1. Field of the Invention
The present invention relates to a metal object detecting apparatus and to a method of detecting a metal object. The invention relates specifically, but not exclusively, to an apparatus for locating, classifying and identifying metal objects carried on a person and to a corresponding method.
2. Description of the Related Art
Metal detectors are used extensively in security critical applications for detecting concealed weapons, for example knives and guns. Such security metal detectors typically comprise a surveillance zone defined by a walk-through archway or portal within which magnetic fields are monitored. A characteristic magnetic field within the surveillance zone is indicative of the presence of a metal object within said area. Passive security metal detectors measure disturbances in the earth's magnetic field due to movement of ferrous metal objects therein, see for example U.S. Pat. No. 6,133,829. Alternatively, active metal detectors transmit a primary, time-dependent magnetic field within the surveillance zone and measure secondary magnetic fields arising from eddy currents induced in any metal object within a zone of influence of the primary magnetic field. Active metal detectors fall broadly into two categories; namely pulse-induction (p.i.) detectors which utilise a transient magnetic field, and continuous wave (c.w.) detectors which use an alternating (sinusoidal) magnetic field.
At their simplest, security metal detectors merely provide an indication of the absence or presence of a metal object within the surveillance zone by comparing the magnitude of the measured magnetic field against a threshold set by the user. In the event that a metal object is detected, the person being screened may have to undergo a thorough search in order to determine the location of the metal object on the person.
More sophisticated security metal detectors are capable of providing an approximate indication of the location of a metal object within the surveillance zone, for example using several transmit and receive coils arranged in zones as described in U.S. Pat. No. 5,859,532.
However, there is an increasing requirement for security metal detectors to be able to provide some form of discrimination between threat items (knives, guns etc.) and non-threat items such a personal electronic devices. A small object can be characterised by its magnetic polarizability tensor (L D Landau and E M Lifschitz, “Electrodynamics of Continuous Media” Pergamon Press, 1960 p. 192) for a given illuminating field frequency (for c.w.) or time delay (p.i.). The polarizability tensor of an object when referred to the frame of reference given by its principal axes is a unique property of that object and can be used to classify or identify it. We will refer to all or part of the magnetic polarizability tensor, or its generalisation for large targets, measured at one or more frequencies (c.w.) or time delays (p.i.) as the magnetic signature of the object.
In order to determine the magnetic signature of an object it is necessary to measure its response to a known magnetic field applied in three linearly independent, preferably substantially orthogonal, directions. Because magnetic field lines are curved, to do this over an extended region of space it is necessary to be able to locate the target in three dimensions. The provision of precise location information and effective discrimination provides benefits in terms of reducing nuisance alarm rates, enabling operators to rapidly resolve potential threats, and even offers the prospect of detection systems having remote supervision or autonomous systems which function without an operator.
International Patent Application Publication No. WO 00/00848 describes an advanced method for locating a metal object within a surveillance zone based on an approximation that the metal object behaves as an oscillating dipole source. The metal detector described in WO 00/00848 measures magnetic field gradients within the surveillance zone and applies an inversion algorithm to said gradients in order to estimate of the location of the dipole source within the surveillance zone.
WO 00/00848 also offers a useful level of discrimination between threat and non-threat items by providing an indication of the magnetic signature of a detected metal object. However, the system described in WO 00/00848 cannot be guaranteed to classify or identify metal objects at substantially all points within the surveillance volume, but is intended to be moved by the user so that a reliable signature can be measured. The system described in WO 00/00848 is not therefore optimised for use in a security metal detector portal.
An alternative approach to precisely locating a single ferromagnetic object in a portal is reported by Perry et al. Proc. SPIE 5071, 362 (2003). However, this approach is incapable of detecting non-ferromagnetic objects or multiple objects, and characterises an object (approximately) only in terms of its permanent magnetic moment. This quantity will vary from object to object, even for nominally identical objects, and offers no prospect of classifying or identifying the object.
Accordingly, it is an object of the invention to provide a metal object detecting apparatus and a method of detecting a metal object which mitigate at least some of the disadvantages of the conventional metal detectors and detection methods described above. It is a further object of the invention to provide an improved apparatus and method for locating, classifying and identifying metal objects.